The proliferation of modern wireless communications devices, such as cell phones, smart phones, and tablet devices, has seen an attendant rise in demand for large multimedia data capabilities for large populations of user equipment (UE) or mobile stations. These multimedia data can include streaming radio, online gaming, music, and TV at the UE. To support this ever increasing demand for higher data rates, multiple-access networks are being deployed based on a variety of transmission techniques such as time division multiple access (TDMA), code division multiple access (CDMA), frequency division multiple access (FDMA), orthogonal frequency division multiple access (OFDMA), and single carrier FDMA (SC_FDMA). New standards for wireless networks are also being developed. Examples of these newer standards include Long Term Evolution (LTE) and LTE-Advanced (LTE-A) being developed by the third generation partnership project (3GPP), the 802.11 and 802.16 family of wireless broadband standards maintained by the Institute of Electric and Electronic Engineers (IEEE), WiMAX, an implementation of the IEEE 802.11 standard from the WiMAX Forum, as well as others. Networks based on these standards provide multiple-access to support multiple simultaneous users by sharing available network resources.
Many of these newer standards support multiple antennas at both the base stations and the UE. These multi antenna configurations are referred at multi-input multi-output (MIMO) and provide improved spectral efficiency resulting in increased data rates. However these improvements come with a cost of increased complexity and computational requirements at the transmitter and receiver.
Increased demand for wireless service has prompted the installation of small cells to enhance coverage of cellular networks. A small cell is a base station or eNodeB having reduced power and coverage area and are often referred to as picocells or femtocells to indicate their reduced power levels as compared to the typical macro cell or macro eNodeB used in conventional cellular networks. A small cell may be used to extend or enhance coverage area such as inside large buildings, or they may be used to add network capacity in high traffic areas such as shopping malls, or train stations. Communication networks that include a mix of macro cells and small cells may be referred to as a heterogeneous network.
Wireless communication networks such as a heterogeneous network include multiple base stations to support downlink and uplink communications with multiple UE. Information sent from a UE to a base station is referred to as uplink communication, and information sent from a base station to a UE is referred to as a downlink communication. In a heterogeneous network the coverage area of small cells often overlaps the coverage area of other cells in the network causing uplink and downlink transmissions to be interfered by transmission of other cells, UE, or RF transmitters. This interference, if not properly accounted for in receivers, can significantly degrade performance of a communication network.
Interference and other distortions in the received signal can be accounted for by creating estimates of the radio channel and using these estimates to support detection of data symbols at the receivers. A common method of obtaining these estimates is to send known symbols, referred to as pilot symbols or reference symbols, in predetermined resource elements interspersed with the transmitted data. A resource element (RE) is the smallest useable portion of the radio spectrum consisting of one sub-carrier during one symbol period and has dimensions of frequency and time. Estimates of the radio channel can be created based on the received signal and knowledge of the pilot symbols. The pilot symbols sent by a particular base station may be referred to as a reference signal and the reference signal sent by all the base stations may be collectively referred to as a common reference signal (CRS). The CRS allow a receiver to determine channel estimates for both its primary data channel, the radio channel over which it is receiving data, as well as any interfering channels.
The reliability with which a receiver can accurately detect the transmitted data symbols is highly dependent on accurate estimation of the radio channel. Estimation of the primary radio channel which is the radio channel between the receiver and the transmitter from which the receiver is receiving its data, referred to herein as the data channel, improves a receiver's ability to detect the data symbols being transmitted. Estimations of radio channels between the receiver and interfering transmitters, referred to herein as the interfering channel, allows the receiver to compensate for distortion of the data signal caused by the interfering transmitters. Better interfering channel estimation results in improved data detection. Thus there is a need for improved methods and apparatus for estimating interfering radio channels in wireless communication networks.